1. Field of the Invention
The present invention relates to a uni-morph, bi-morph or other types of piezoelectric or electrostrictive elements which undergo displacement in the form of bending, deflection or flexure and which can be used for ink jet print heads, microphones, sounding bodies (such as loudspeakers), various resonators or vibrators, sensors, and other components or devices. The term "element" used herein is interpreted to mean an element capable of transducing or converting an electric energy into a mechanical energy, e.g., mechanical force, displacement, strain or vibrations, or transducing such a mechanical energy into an electric energy.
2. Discussion of the Related Art
In the recent years, in the fields of optics and precision positioning or machining operations, for example, there have been widely used and increasingly demanded an element whose displacement can be controlled for adjusting or controlling an optical path length or the position of a member or component of a device, on the order of fractions of a micron (.mu.m), and a detecting element adapted to detect infinitesimal displacement of a subject as an electric change. To meet this need, there have been developed various piezoelectric or electrostrictive elements utilizing the reverse or converse piezoelectric effect or electrostrictive effect in which a piezoelectric or electrostrictive material produces a mechanical displacement upon application of a voltage or electric field thereto, or the piezoelectric effect in which the piezoelectric material produces a voltage or electric field upon application of pressure or mechanical stress. Recently, there are increasing requirements for further improved capability and operating reliability of the piezoelectric/electrostrictive elements.
The piezoelectric or electrostrictive element of the uni-morph or bi-morph type is carried by a substrate in the form of a plate, which functions as an oscillator or transducer plate. For assuring sufficient amount and force of bending or flexural displacement of the substrate or plate, it is important to reduce the thickness of the substrate, on the one hand.
On the other hand, however, the reduction in the substrate thickness undesirably results in a decrease in the mechanical strength of the substrate. In addition, the piezoelectric or electrostrictive element of the conventional uni-morph or bi-morph type is unsatisfactory in its operating reliability since the element consists of mutually superposed plate-like members including a piezoelectric/electrostrictive plate, which members are bonded to each other by means of adhesives, for example.
In view of the above problems, the inventors of the present invention developed a piezoelectric/electrostrictive element as disclosed in co-pending U.S. patent application Ser. No. 07/912,920, which includes a ceramic substrate formed principally of zirconia whose crystal phase is partially or fully stabilized. On this ceramic substrate are formed in lamination a lower electrode, a piezoelectric/electrostrictive layer and an upper electrode, by a suitable film-forming technique. The piezoelectric/electrostrictive layer is formed of a piezoelectric/electrostrictive material having a composite perovskite structure which contains lead element.
The piezoelectric/electrostrictive element as described just above can be formed with a significantly reduced thickness as compared with the conventional counterpart, and is therefore able to produce a relatively large amount of displacement by application of a relatively low voltage thereto, with a sufficiently large magnitude of force or electric potential generated, assuring an improved operating response. Further, the use of the ceramic substrate formed principally of partially or fully stabilized zirconia leads to a significantly increased mechanical strength of the element.
In the piezoelectric/electrostrictive element of the above type, it is desirable that electrical lead portions of the lower and upper electrodes are both formed on the ceramic substrate, so as to facilitate electrical connection of the electrodes with an outside electric circuit. One considered example, such a desirable piezoelectric/electrostrictive element is constructed such that an upper electrode 6 is formed on a piezoelectric/electrostrictive layer 4 formed on a ceramic substrate 2, with an electrical lead portion of the electrode 6 formed on the substrate 2, as shown in FIG. 6, wherein reference numeral 8 denotes a lower electrode whose lead portion is also formed on the substrate 2.
However, it is difficult for this type of element to assure a sufficiently great bonding strength between the piezoelectric/electrostrictive layer 4 and the ceramic substrate 2, since desired piezoelectric/electrostrictive characteristics of the layer 4 can be maintained by restraining or reducing the chemical reaction between the layer 4 and the substrate 2. Also, the piezoelectric/electrostrictive layer 4 should not be bonded or joined to the ceramic substrate 2 in terms of the mechanism of displacement or deformation of the element.
If the upper electrode 6 is formed to extend between the piezoelectric/electrostrictive layer 4 and ceramic substrate 2 which are not joined together, the upper electrode 6 tends to be disconnected at the interface between the ceramic substrate 2 and the layer 4, due to heat, mechanical shock, vibrations and the like which may occur during manufacture or use of the element. This makes it difficult for the element to assure sufficiently high operating reliability.
To deal with the above problem, it is proposed to provide a coating layer made of resin, for example, at the interface between the substrate 2 and the piezoelectric/electrostrictive layer 4. However, this method was found unsatisfactory to solve the above problem.